A piston for an internal combustion engine which is coupled to a connecting rod by a spherical joint is known from, for example, JP-A-9-144879 and JP-A-2000-213646. JP-A-9-144879 will be explained with reference to FIG. 16 hereof and JP-A-2000-213646 with reference to FIG. 17 hereof.
Referring to FIG. 16, a piston 200 has a concave spherical washer 203 having a concave spherical surface 202 and held on the rear surface of a crown 201 with a retainer 204, a connecting rod 206 having a convex spherical surface 208 formed on a small end 207 thereof and fitted sidably in the concave spherical surface 202 of the washer.
The concave spherical washer 203 and the retainer 204 on the rear surface of the crown 201, however, add to the weight of the piston 200. A reduction of its weight is difficult, since the concave spherical washer 203, retainer 204, bolts 211 and nuts 212 occupy the space under the crown 201 and do not allow the thickness of the crown 201 to be reduced.
As the concave spherical washer 203 is made of an iron-based material while the piston 200 is of an aluminum alloy, the concave spherical washer 203 is so low in thermal conductivity that heat is not conducted from the crown 201 to, for example, the connecting rod 206 easily. Accordingly, the crown 201 has a high temperature, raising the temperature of a land 214 and a skirt 215 near ring grooves adjacent to the crown 201 and thereby increasing the thermal expansion of the land 214 and the skirt 215. This makes it impossible to secure a proper clearance between the piston 200 and the wall of a cylinder bore in which the piston 200 is slidable.
Referring now to FIG. 17, a piston 220 has a spherical recess 221 formed in the rear surface of its crown 220a, while a connecting rod 222 has a spherical top 223 fitted in the spherical recess 221 and having a lower portion supported sidably on the spherical inner surface 226 of a fixing plate 224 secured to the rear surface of the crown 220a of the piston 220 by bolts 227.
The crown 220a of the piston 220 has a large thickness, since its rear surface is situated at as low a level as the vertically middle portion of the top 223 of the connecting rod 222 so that the fixing plate 224 may be secured thereto. A reduction in thickness of the crown 220a, for example, would enable a reduction in weight of the piston. It would, however, be necessary to prevent the concentration of stress on the crown 220a from being caused by the pressure of combustion gases and force of inertia acting upon the piston 220.
An elevation in temperature of the crown 220a resulting from engine operation is likely to lower the viscosity of the lubricant oil between the recess 221 of the piston 220 and the top 223 of the connecting rod 222 and eventually cause the oil film to disappear from therebetween. The same reason as has been stated in connection with JP-A-9-144879 disables a proper clearance to be secured between the piston 220 and a cylinder. An increase in the dissipation of heat from the crown 220a would make the coupling between the piston 220 and the connecting rod 222 and the sliding of the piston 220 relative to the cylinder possible more smoothly.
As the pressure of combustion gases acts upon the piston for an internal combustion engine during its combustion stroke, an external force produced by the pressure of combustion gases acts upon various parts of the piston. This external force will be explained with reference to FIG. 18.
Referring to FIG. 18, a piston 232 is movably mounted in a cylinder 231 and a connecting rod 234 has a small end 236 attached to the piston 232 rotatably by a piston pin 233. The connecting rod 234 also has a big end 237 linked to a crank pin on a crankshaft. Reference numerals 241, 242 and 243 denote the crown, land and skirt, respectively, of the piston 232, 244 denotes a combustion chamber, 246 is a point indicating the axis of the crank pin, 247 is a point indicating the axis of the crankshaft and 248 indicates the axis of the cylinder 231.
Upon explosion of a fuel mixture in the combustion chamber 244, the pressure of combustion gases acts downwardly upon the top 241 of the piston 232 as shown by an outline arrow, and as an upward force of inertia acts upon the piston 232 when it moves down, the resultant of the forces of combustion gases and inertia, which is equal to the force of combustion gases less the force of inertia, acts upon the piston 232 and an upward force F equal to the resultant is produced. When the connecting rod 234 is inclined by an angle θ to the axis 248, a thrust R (=F·tan θ) is produced as a component force.
That side of the piston 232 on which the thrust R is produced is called the thrust side, which lies in FIG. 18 on the left-hand side of a plane extending through the axis of rotation of the connecting rod 234 relative to the piston 232 (i.e. the axis of the piston pin 233 extending at right angles to the drawing sheet and indicated by a point 250) and in parallel to the axis 248, while the right-hand side of the plane in FIG. 18 is called the anti-thrust side.
There is known a piston for an internal combustion engine in which a rib formed on the rear surface of its crown has a different shape between its thrust and anti-thrust sides, as disclosed, for example, in JP-A-2000-97105. The following is an outline of its disclosure.
The piston has a head, a pair of pin bosses protruding from the head and a substantially arcuate skirt extending from the edge of the head. The piston also has a front and a rear rib joining the pin bosses and skirt integrally with the lower surface of the head.
The pressure of combustion acting upon the head, for example, produces a large stress thereon. An increase in thickness of the head may be effective for reducing any such stress, but as the rear surface of the head is not simple in shape because of the pin bosses, skirt, etc., the stress is concentrated on local areas of the head and is difficult to reduce by increasing the thickness of the head uniformly across its rear surface. Moreover, an increase in thickness of the head adds to the weight of the piston, i.e. its inertia weight, thereby making it difficult to obtain a piston suitable for an internal combustion engine of faster rotation and higher output.
Therefore, it is desirable to reduce any concentration of stress on the crown of a piston for an internal combustion engine, achieve a reduction in weight of the piston and a lower piston temperature and thereby realize an internal combustion engine of faster rotation and higher output.